Rubber articles designed to withstand considerable stresses in use are reinforced with plies of comparatively inextensible textile materials. The rubber hose, belts and tires are commonly reinforced with filamentary textiles in the form of yarns, cords or fabric. In the articles, it is desirable that the plies of textile reinforcing material be firmly adhered to the rubber between the textile plies and remain effectively adhered even after the article has been subjected to repeated varying strains in use. Any separation and relative movement of the rubber and textile parts lead to abrasion between these parts and result in failure.
The majority of rubber articles in which high tensile strength is required are reinforced with textile materials such as rayon, nylon, aramid and polyester fibers. Polyester fibers, based on poly(ethylene terephthalate), namely (PET), have superior physical and chemical properties and are also produced in large quantities. Polyester's high modulus and low elongation, particularly in the tire application, reduce tire deformation and lead to better high speed performance and tread wear, reduced stress cracking and better steering characteristics.
Polyester is a very ideal material for use as the reinforcing material in the manufacture of various rubber articles including the tire. But it is relatively difficult to achieve a good bond between polyester and rubber. The poor bondability of polyester with rubber is attributed to the hydrophobic nature of fiber surface due to the limited number of polar or active hydrogen bonding groups, such as the hydroxyl (—OH) and carboxyl (—COOH), at the end of polyester molecule. Consequently, when a simple adhesives composition is applied to polyester surfaces to improve the bond with rubber, the results are poor due to poor compatibility. Molecular inter-diffusion between the adhesive and substrate (namely the fiber or rubber) is an important factor in the adhesion development of polyester material. Diffusion and subsequent interaction are much more favored when the adhesive and substrate are thermodynamically compatible.
Even though the resorcinol or resorcinol-formaldehyde (R/F) based adhesive formulations are now used for improving the bonding performance of various synthetic fiber materials to rubber compounds, constant adhesion improvements needed in these materials for various high performance applications require higher performance resorcinolic products for use. This is particularly true for providing a resorcinolic novolak resin solution for an application of the most difficult adhesion enhancement between the polyester fibers and rubber compounds. Therefore, there is a need for a flexibilized resorcinolic novolak resins that are useful in the adhesive formulations development for improving or enhancing the polyester tire cords adhesion towards the rubber compounds used by the tire industry.